The invention relates generally to signal processing for global positioning systems. In particular, the invention relates to using space-frequency adaptive processing (SFAP) for providing anti-jam functionality. More particularly still, the invention relates to computational enhancements of space-frequency adaptive processing for GPS anti-jamming applications.
Global navigation satellite system (GNSS) receivers, such as global positioning system (GPS) receivers, receive and process multiple satellite positioning signals simultaneously. Frequently, GNSS receivers operate in the presence of interfering or jamming signals. Generally, in the presence of such interfering and jamming signals, signal processing techniques such as space-frequency adaptive processing using fast Fourier transforms (FFTs) may be used to filter the jamming signals from the positioning signals.
Fast Fourier transforms (FFTs) are used for processing observed signals. Weighting functions, referred to as windows, are applied to the data to reduce the spectral leakage associated with the finite observation intervals. In the SFAP algorithm, an N point FFT of input data is taken. In each frequency bin, a set of adaptive weights is calculated to remove the jamming power in a given bin. The weights can be chosen optimally by combining the effects of every bin (as there is bin leakage in an FFT) in the weight solution, however typical (and computationally feasible) implementations calculate the weight for a given bin using only the data from the bin. When not jointly optimizing each of the bins, typically windowing is required before the FFT to reduce leakage between the bins. Windowing achieves the effect of attenuating the contribution from bins located several bins and beyond away from the current bin. This is essential for high anti-jam processing as the leakage would look like additional jammers which would significantly reduce the nulling performance of the system. Attenuation of outlying bins is achieved at the cost of xe2x80x9csmearingxe2x80x9d adjacent bins (increasing the correlation between the adjacent bins) due to widening of the window""s main lobe.
One of the inhibiting factors for a practical implementation of digital high anti-jamming technologies for GPS is the large computational burden imposed on the digital signal processor (DSP) by the SFAP algorithm which may be necessary to achieve high (greater than 50 dB) anti-jam protection. The high computational burden adds to increased system costs as an expensive high-end DSP is needed, requires significant power consumption in the DSP, and may even prevent an algorithm from being implemented due to availability of a capable DSP if cost and power are not an issue. For a space-frequency implementation of the adaptive GPS anti-jam processing, it would be desirable to use an algorithm that has been developed to reduce the computational load on the DSP. Further, it would be desirable to provide an algorithm that reduces the computational load on the DSP by a factor of two, yet does not sacrifice performance against broad band jammers and achieves comparable performance for continuous wave (CW) jammers.
It would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.
An example of the invention relates to a signal processing system. The signal processing system comprises an antenna receiving a radio frequency (RF) signal, radio frequency circuitry coupled to the RF antenna, and an analog to digital converter coupled to the RF circuitry and converting an analog signal from the RF circuitry to a digital signal. The signal processing system also comprises a processing device generating a fast Fourier transform including N bins. Further, the signal processing system comprises an adaptive weight calculator calculating no more than (N/2)+1 weights based on information from at least (N/2)xe2x88x921 bins, the no more than (N/2)+1 weights being applied to the output of the N bins in a weighted summation, and an inverse fast Fourier transform calculator producing an inverse fast Fourier transform using the weighted summation.
Another example of the invention relates to a method of providing anti-jamming processing. The method comprises receiving a digital input signal. The method also comprises performing a fast Fourier transform with an output of N frequency bins. The method further comprises calculating a set of adaptive weights based on the output of at least some of the bins, wherein the number of adaptive weights calculated is not more than (N/2)+1. Further still, the method comprises duplicating not more than (N/2) of the not more than (N/2)+1 weights, and applying the not more than (N/2) weights and the not more than (N/2)+1 alternately to adjacent bins.
Yet another example of the invention relates to a global positioning system device. The global positioning system device comprises an antenna receiving a radio frequency (RF) signal, radio frequency circuitry coupled to the RF antenna, and an analog to digital converter coupled to the RF circuitry and converting an analog signal from the RF circuitry to a digital signal. The global positioning system device also comprises a processing device generating a fast Fourier transform including N bins. Further, the global positioning system device comprises an adaptive weight calculator calculating no more than (N/2)+1 weights based on information from at least (N/2)xe2x88x921 bins, the no more than (N/2)+1 weights being applied to the output of the N bins in a weighted summation, and an inverse fast Fourier transform calculator producing an inverse fast Fourier transform using the weighted summation.
Alternative examples and other exemplary embodiments relate to other features and combination of features as may be generally recited in the claims.